Examville.com - Bacterial Genetics, Structure Of Rna, Dna Re Pi Cation

Bacterial Genetics Structure of RNA Ribonucleic acid (RNA), the second principal kind of nucleic acid, differs from DNA as RNA is usually single stranded, the five carbon sugar in the RNA nucleotide is ribose as compared to deoxyribose in DNA. One of the RNA's bases is uracil (U) instead of thymine. The other 3 bases (A, G and C) are the same in as DNA. At least 3 kinds of RNA-ribosomal RNA, messenger RNA and transfer RNA are involved in protein synthesis. Gene can be defined as a segment of DNA (sequence of nucleotides in DNA) that codes for a functional product. A DNA molecule consists of a large number of genes each of which contains hundreds of thousands of nucleotides. The length of DNA is usually expressed in kilobases (1 kb = 1000 base pairs) and bacterial DNA is about 4000 kb in length. The genotype of an organism is its genetic make up, the information that codes for all particular characteristics of the organism. Phenotype refers to the actual, expressed properties. Phenotype is thus the manifestation of the genotype.

DNA Replication A semiconservative model for the replication of DNA ensures transmission of genetic information present in the parent is faithfully transmitted to the progeny. It also means that after one generation DNA is present in a hybrid form which contains half old and half new DNA. Replication of DNA occurs at a growing point (fork) that moves linearly from an origin to a terminus usually in both the directions. Genetic code is the information which resides in the nucleic acids of the organisms. From DNA it is passed onto mRNA through which it is translated into the primary structure of proteins.

The extrachromosomal genetic elements, called as plasmids are autonomously replicating, cyclic, double stranded DNA molecules which are distinct from the cellular chromosome. The plasmids carry genes that are not essential for host cell growth while the chromosome carries all the necessary genes. Properties of plasmids are: · Autonomously replicate in host cell · Plasmid specificity is shown by host · May express phenotypically · Some may have apparatus for transfer · Can reversibly integrate into host chromosome · Can transfer even chromosomal genes · Free DNA is transferred by transfection Bacterial Variations Phenotypic Variations are acquired during life of a bacterium and may not be passed down to progeny. Genetic Variations influence the genetic composition of the bacterium and are transmitted to the next generation. The genetic variations can be due to two reasons: Alterations in the genome structure due to mutations and Acquisition of genetic material through gene transfer. MUTATIONS Mutation can be defined as any change in the sequence of bases of DNA, irrespective of detectable change in the cell phenotype, may be spontaneous or induced by mutagenic agents. Those mutations which do not express phenotypically are known as silent mutations. Point mutation consists of a change in a single nucleotide. Frame shift mutation consists of the insertion or deletion of a single nucleotide. Among the replacements, missense mutation causes one amino acid to replace another and the resultant protein may retain its function without any major change in tertiary structure or active sites. An altered enzyme function due to mutation may result into the whole organism becoming temperature sensitive. Nonsense mutations create a codon that prematurely terminates the growing peptide chain and almost always destroys the function of the proteins. Induced mutations are mutations produced by agents called mutagens. These are: 1. Agents which alter the pyrimidines or purines so as to cause error in base pairing. 2. Agents which interact with DNA and its secondary structure producing local distortions in the helix thus giving rise to errors of replication. 3. Base analogs which are incorporated into the DNA and cause replication errors. Mutation Rate The mutation rate is the probability of a gene mutating each time a cell divides. Ames Test It is used to test whether a particular substance can induce mutations or not. Ames test is based on the hypothesis that if a substance is a mutagen, it will increase the rate at which these organisms revert to histidine synthesizers.